Fixed #36388 -- Made QuerySet.union() return self when called with no arguments.

Regression in 9cb8baa0c4.
Thank you to Antoine Humeau for the report and Simon Charette for the review.

AUTHORS

@@ -239,6 +239,7 @@ answer newbie questions, and generally made Django that much better:
     Claude Paroz <claude@2xlibre.net>
     Clifford Gama <cliffygamy@gmail.com>
     Clint Ecker
+    Colleen Dunlap <https://medium.com/@colleen85052>
     colin@owlfish.com
     Colin Wood <cwood06@gmail.com>
     Collin Anderson <cmawebsite@gmail.com>

django/db/models/query.py

@@ -1550,6 +1550,8 @@ class QuerySet(AltersData):
             if len(qs) == 1:
                 return qs[0]
             return qs[0]._combinator_query("union", *qs[1:], all=all)
+        elif not other_qs:
+            return self
         return self._combinator_query("union", *other_qs, all=all)
 
     def intersection(self, *other_qs):

docs/releases/5.2.2.txt

@@ -15,3 +15,6 @@ Bugfixes
 * Fixed a bug in Django 5.2 where subqueries using ``"pk"`` to reference models
   with a ``CompositePrimaryKey`` failed to raise ``ValueError`` when too many
   or too few columns were selected (:ticket:`36392`).
+
+* Fixed a regression in Django 5.2 that caused a crash when no arguments were
+  passed into ``QuerySet.union()`` (:ticket:`36388`).

tests/queries/test_qs_combinators.py

@@ -88,6 +88,13 @@ class QuerySetSetOperationTests(TestCase):
         qs3 = qs1.union(qs2)
         self.assertNumbersEqual(qs3[:1], [0])
 
+    def test_union_empty_slice(self):
+        qs = Number.objects.union()
+        self.assertNumbersEqual(qs[:1], [0])
+        qs = Number.objects.union(all=True)
+        self.assertNumbersEqual(qs[:1], [0])
+        self.assertNumbersEqual(qs.order_by("num")[0:], list(range(0, 10)))
+
     def test_union_all_none_slice(self):
         qs = Number.objects.filter(id__in=[])
         with self.assertNumQueries(0):